Ataxia-Telangiectasia (A-T) is rare (~ 1 in every 100,000) but a catastrophic and deadly disease that is often caused by a nonsense mutation (that is, a premature termination codon [PTC]) in the Atm (Ataxia- Telangiectasia mutated) gene. Children born with A-T suffer a progressive loss of motor function and coordination, become wheelchair-bound, and typically die by age 25. No effective treatments are available, in part because no satisfactory animal model exists. Atm-/- mice are not a good model because they do not develop ataxia. However, recognizing that the Atm gene product participates in DNA repair pathways, our collaborator Dr. McKinnon reasoned that inflicting a second hit ? an additional knockout mutation in a second DNA repair gene ? might produce ataxia. To test this notion, he selected to knockout the Aptx (aprataxin) gene because: 1) APTX participates in DNA repair: 2) humans lacking APTX protein develop a disease similar to A-T called AOA (Ataxia with Oculomotor Apraxia); and 3) like Atm-/- mice, Aptx-/- mice do not develop ataxia. He crossed the single mutant mice to generate a double mutant Atm-/-; Aptx-/- mouse that, as anticipated, exhibits the progressive ataxia observed in A-T and AOA patients. Our LONG TERM GOAL is to utilize these mice to test the safety and effectiveness of potential treatments for A-T and deploy them for clinical use. However, due to the nature of the genetic mutation in these mice (total knockout of the Atm gene), this model is not suitable for testing our recently developed Small Molecule Read-Through (SMRT) compounds, which efficiently read- through the mutation in Atm genes causing the PTCs in A-T (Lee et al., 2013). We have synthesized, purified, and patented a number of derivatives from our original screen that read through all three types of PTCs (TAG, TAA, and TGA) with similar efficiency in Atm and other PTC-containing genes as well as cross the blood brain barrier. To test whether SMRT compounds effectively treat A-T caused by a PTC, we need to create and validate a mouse model of A-T that incorporates a patient-derived PTC and displays ataxia. As a first step, we have created a mutant mouse that expresses a PTC in the Atm gene at the location on exon 15 that it occurs in many patients with A-T. These mice are viable and fertile. Like the Atm knockout, this AtmN/N genotype (?N? for nonsense mutation) is not ataxic. To induce ataxia, we will cross AtmN/N mice with Aptx-/- mice. We predict that like the Atm-/-; Aptx-/- mice, the resultant AtmN/N; Aptx-/- mice will exhibit ataxia and will also contain a PTC amenable to treatment with our SMRT compounds. We propose here to breed, genotype, characterize, and phenotype AtmN/N; Aptx-/- compound mutant mice. FUTURE DIRECTIONS: Once the AtmN/N; Aptx-/- mice are fully phenotyped, we will then proceed to perform safety and efficacy studies for our SMRT compounds. This treatment should restore production of ATM protein to sufficient quantities to normalize (or at least alleviate) the symptoms of A-T, including ataxia, immunodeficiency, and high susceptibility to cancer. Our approach could in principle be applied to hundreds of inherited human diseases that are similarly caused by a PTC mutation.